The present invention relates generally to monitoring electrical circuit components and more particularly to a method and apparatus for continuously monitoring the operational condition of electrical circuits which may provide the same output in response to a normal operating condition and to a failed circuit condition.
In certain instances, electrical circuitry such as control circuitry may provide the same output in response to a normal operating condition and a failure condition. As an example, in certain amplifying control circuitry there may be a zero output in response to a zero input, indicating that there is no extant error condition, and the same zero condition may also exist if the circuit were to fail. In such situations, it is often difficult to determine that this circuit or component is functionally operative. It is possible, of course, to periodically apply a test signal and to check that the output is of a normal value. This method, however, has two basic problems. The first of these problems is that if the circuit is in a direct feedback loop, it must be disconnected from that loop in order not to disturb the system being controlled. The second major drawback is that this type of an arrangement does not provide for the continuous monitoring of the circuit or component.
As an example in which the above situation exists, attention is called to the U.S. Pat. No. 4,855,644 "Method and Apparatus for Damping Oscillations of AC Generator" by L.J. Lane, issued Aug. 8, 1989. As recognized in that patent, the speed of a given generator can oscillate above or below its synchronous speed such that its period of oscillation is in the approximate range of 0.2 to 2.0 seconds. It is known that the generator can effect very little damping to electromechanical oscillations which can result in spontaneous oscillations or even loss of synchronism with the power system and the attendant large voltage differentials. Mechanical oscillation and loss of synchronism may affect the operational life of the generator and the reliability of the power system. This patent describes a polyphase system in which each phase includes circuitry, including an amplifier, for controlling the generator field excitation. Since the generator may remain in a quiescent state not requiring any change in generator excitation through a power system stabilizer, it is seen that the output of the circuitry could remain at zero for long periods of time. The concern with this system is that because the generator may remain in the quiescent state for comparatively long periods of time with no change in output signal, there is no assurance that the circuitry is continuously operable.